Curved Smectite in Soils from Volcanic Ash in Kenya and Tanzania: A Low-Angle X-ray Powder Diffraction Study

S. J. van der Gaast1, Chitoshi Mizota2 and J. H. F. Jansen1
1 Netherlands Intitute for Sea Research, P.O. Box 59 1790 AB Den Burg, Texel, The Netherlands
2 Faculty of Agriculture, Kyushu University 46-02 Fukuoka 812, Japan

Abstract: Low-angle X-ray powder diffraction (XRD) measurements of soil samples, made at controlled relative humidities, showed the presence of major reflections at 20–33, 10–27, and 7–8 Å. The first reflection, which increased in intensity but did not shift in spacing with decreasing relative humidity, represents curved smectite layers. This spacing was also observed by high-resolution transmission electron microscopy. The value of 10–27 Å for the second reflection, the 001 reflection of smetite, is unusually high, probably due to poorly stacked, irregularly curved layers. The 7–8-Å reflection originates from kaolinite or dehydrated halloysite, which also contain curved layers. The more curved the layer structure of the smectite, the more difficult it is to detect this phase; therefore the XRD relative peak heights are not directly proportional to the percentages of the smectite.

Key Words: Curved layers • Kaolinite • Low-angle X-ray powder diffraction • Smectite • Soil clay • Relative humidity • Volcanic ash

Clays and Clay Minerals; December 1986 v. 34; no. 6; p. 665-671; DOI: 10.1346/CCMN.1986.0340607
© 1986, The Clay Minerals Society
Clay Minerals Society (www.clays.org)